pgsql: Some minor further cleanup around A_Const.

Log Message:
-----------
Some minor further cleanup around A_Const. Don't attach a typecast in
makeFloatConst, and avoid "manual" construction of A_Const nodes in grammar
productions, in favor of using makeXXXConst subroutines.

Modified Files:
--------------
pgsql/src/backend/parser:
gram.y (r2.613 -> r2.614)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y?r1=2.613&r2=2.614)
parse_type.c (r1.96 -> r1.97)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_type.c?r1=1.96&r2=1.97)
pgsql/src/backend/utils/misc:
guc.c (r1.448 -> r1.449)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.448&r2=1.449)
pgsql/src/include/nodes:
parsenodes.h (r1.363 -> r1.364)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/parsenodes.h?r1=1.363&r2=1.364)